This invention relates generally to rocket propulsion systems and more particularly, to rocket propulsion systems for placing and maintaining spacecraft in planetary orbits. Although the invention has broad application to unmanned spacecraft, it is particularly concerned with launching and maintaining satellites in geosynchronous orbits. Placing a geosynchronous satellite into orbit typically involves three principal mission phases. First, the satellite is placed in the low earth orbit not far above the earth's atmosphere, either as a part of payload of the space shuttle vehicle or on a conventional non-reusable rocket vehicle. In the second phase, the satellite orbit has its apogee or highest point raised in altitude by one or more rocket "burns" at a select point in orbit, until the apogee is approximately at geosynchronous altitude. Finally, the satellite is given an apogee "kick," i.e., a further rocket burn at apogee that circularizes the orbit at geosynchronous altitude.
Once in orbit, rocket engines will be called on for two further functions: station keeping and attitude control, which are sometimes referred to collectively as reaction control systems (RCS) functions. Satellites are usually required to maintain a particular "station" with respect to the earth's surface. Maintaining this station requires the expenditure of energy, even though the orbit is theoretically self-sustaining and geosynchronous. Various factors, such as the non-spherical nature of the earth, gravitational influences of the moon and sun, and so forth, require that the orbit be corrected from time to time if the required station is to be maintained. Attitude control is simply the use of multiple rocket engines on the spacecraft to maintain a particular angular attitude of the vehicle. This may be needed, for example, to point an antenna or other sensor at the earth, the sun, or a star.
Rocket engines associated with orbiting spacecraft may be called upon to perform the various functions of orbital transfer, station keeping and attitude control. Unfortunately, the performance characteristics required for these functions are not identical. A figure of merit often used in comparison of rocket engines is the specific impulse, I.sub.sp, which is defined as the thrust developed by an engine per unit of propellant weight flow rate. If the thrust is measured in pounds and the flow rate in pounds per second, then the units of measurement of specific impulse are seconds. The specific impulse is analogous to miles-per-gallon figure for an automobile, since it measures how much thrust is developed for a unit fuel flow rate.
Another measure of performance is, of course, the thrust force generated by the engine. For the rapid acceleration as required in transition to geosynchronous orbit, particularly at the apogee "kick" phase of the mission, an engine with a relatively large thrust is required, perhaps generating up to several thousand pounds of thrust force. The specific impulse is also important, and should be in the 300-400 second range. For station keeping and attitude control, high thrust is not quite so important, since most station keeping and attitude control maneuvers can be accomplished with low thrust burns of the rocket engines. However, fuel economy and engine durability is very important for the rocket engines used in these activities. Rockets of this type are used repeatedly over a mission that may last as many as ten years and therefore engine durability is important. Cooling the small engines used for RCS functions is difficult due to their small thermal radiating surfaces. Thus, any prolonged use may melt the thrust chamber. However, durability can be improved by using special materials such as COLUMBIUM, which can withstand the 4000.degree. F. -5000.degree. F. temperatures generated in the combustion chamber.
In the past, the approach followed to launch a satellite has typically involved using multiple fuels and engine systems for two tasks. For example, a solid rocket is used for the apogee kick engine and hydrazine catalytic engines for the station keeping and attitude control system thrusters. There is nothing inherently wrong with this traditional approach, except that the use of two separate propulsion systems severely limits the size and the useful payload that can be placed and maintained in orbit.
Some improvement can be attained using an integrated bipropellant system, in which both the apogee "kick" engine and the RCS thrusters use a bipropellant fuel system, such as monomethyl hydrazine (MMH) as a fuel and nitrogen tetroxide (N.sub.2 O.sub.4) as an oxidizer. However, there is still room for further improvement in the payload that can be placed in orbit for a given mission. Another way to look at the problem is that there is room for improvement in the lifetime that a given spacecraft payload may be maintained in orbit. With a more efficient propulsion system, a greater payload may be maintained in orbit for a given time, or the same payload may be maintained in orbit for a longer time.
The present invention in one embodiment provides for a more efficient propulsion system suitable for geosynchronous and other high energy mission spacecraft programs. The preferred embodiment relates to a cooled bipropellant thruster for controlling the on-orbit position and orientation of a spacecraft.